//寻找缺失的数
public class TakeAttendance {
    public int code(int[] records) {
        int left = 0, right = records.length - 1;
        while (left < right){
            int mid = left + (right - left) / 2;
            if (records[mid] == mid){
                left = mid + 1;
            }else{
                right = mid;
            }
        }
        return left == records[left]? left+1 : left;
    }
}
